Previous devices for punching holes through thin gauge materials, including elastomers, have typically included cylindrical or straight sided punch elements which coact with similarly shaped female die elements to punch out holes in such materials. Various problems arise with the use of straight sided punches which have relatively deep penetration within a coacting die and particularly when blanking out thin gauge elastomeric materials. First, where a straight sided punch penetrates a corresponding straight sided die opening to shear or blank out an intermediate material, a certain clearance must be provided between the punch and die elements. Such clearance is provided to prevent damage and wear as would occur through direct contact between the punch and die. As such clearance increases due either to manufacturing tolerances or wear, there is an increasing tendency of the material being blanked or sheared to be drawn within the clearance space between the punch and die elements. As this occurs, the shearing action becomes less sharp or defined and the material tends to be torn rather than sheared, leaving a somewhat ragged or imprecise hole or edge. As this situation becomes aggravated it is often necessary to manually repair the torn edge or else to reject the part.
The aforementioned problem is accentuated as thinner gauge materials are blanked since the material thickness or gauge begins to approach the clearance between the punch and die. The problem is further aggravated where thin wall elastomeric materials are to be blanked since such materials are stretched through engagement by the punch which further thins the material cross section thereby contributing to tearing and irregularly formed holes. The amount of such stretching will vary directly with the amount of clearance between the punch and die elements. In other words, the greater the amount of clearance between the male and female elements, the greater the amount of stretching. The stretching can cause the elastomeric material to rip or tear rather than being cleanly sheared off with the resultant hole being irregular or having small amounts of material remaining therearound which subsequently have to be removed by hand. Further, such stretching can cause a mislocation of the hole when the elastomeric material contracts after the shearing operation. Such mislocation of holes can be particularly acute in those cases where a plurality of adjacently related holes are being punched through the elastomeric material at the same time. In other words, where multiple punch elements are being utilized and where one or more of such punches, due to the aforementioned clearances between punch and die elements, causes the elastomeric material to be stretched or drawn into a particular die element, then, through such stretching, adjacent sections of the elastomeric material can be laterally displaced thereby causing adjacent holes to be mislocated when the elastomeric material contracts after the punching operation.
Prior experience with the aforementioned straight sided type of cylindrical male and female punch and die elements has shown a relatively high rate of malformed parts requiring either manual correction or total rejection of unusable parts. It has also been found that such rejection or repair rate increases as the punch elements become worn with use. Incidentally, the wear rate for such punches is found to be quite high in shearing such thin sectioned elastomeric materials due to such material being drawn between the die and punch elements.